1. Field of the Invention
The present invention relates generally to a welding fixture for holding in place relatively long components members that are welded along their length into an elongate piece or product, and more particularly to such a welding fixture which is usable for both preliminary welding to prepare a pre-welded assembly of the component members, and main welding applied to the pre-welded assembly to produce the intended welded piece.
2. Discussion of the Prior Art
There are many varied elongate parts or products, each of which consists of a plurality of long component members which are welded together along their length. Generally, the component members of such products are first assembled in a given positional relation with each other, and the assembled members are subjected to a preliminary welding at a plurality of points that are spaced from each other in the longitudinal direction of the assembly. Then, a main welding is applied to the pre-welded assembly, to weld the component members continuously along their length. Thus, the desired welded product is obtained.
For example, a frame of a truck or cargo vehicle includes a pair of elongate parts in the form of right and left side rails 10, and a plurality of cross members 12 which connect the two side rails 10, such that the side rails 10 and the cross members 12 form a ladder-like structure. Each of the side rails 10 consists of two relatively long component members, namely, an outer channel 14 and an inner channel 16, which have U-shaped transverse cross section, as shown in FIG. 11. The inner channel 16 is partially fitted in the opening of the outer channel 14, such that the two channels 14, 16 from an enclosed structure having a rectangular transverse cross section. A suitable spacer 18 is positioned within the enclosed structure to maintain a desired distance between the opposite long sides. In this condition, the overlapping longitudinal portions of the outer and inner channels 14, 16 are welded together by continuously applied welds indicated at 20 in FIG. 11. usually, this continuous welding will cause the welded side rail 10 to be warped, as illustrated in two-dot chain line in FIG. 9, due to welding strains. In the light of this undesirable phenomenon, it is a common practice to give the pre-welded assembly of the channels 14, 16 a suitable amount of bending in the direction opposite to the direction of the welding warpage, before the pre-welded assembly is subjected to the main welding process.
Laid-open Publication No. 60-28994 (published in 1985) of Japanese Patent Application discloses a welding fixture which is adapted to hold a pre-welded assembly in a slightly bent state for compensating for a subsequent welding warpage or deflection. The disclosed welding fixture uses at least three clamping units arranged in the longitudinal direction of the pre-welded assembly. Each clamping unit consists of a stationary clamp, and a movable clamp which is located opposite to the stationary clamp. The stationary and movable clamps cooperate to clamp the pre-welded assembly in opposite directions toward each other, at the corresponding longitudinal portion of the assembly. At least one of the stationary clamps used is supported on a pivot base which is pivotable in a plane parallel to the clamping direction. The pre-welded assembly is bent by a pivotal movement of the pivot base, due to a change in the position of the stationary clamp or clamps supported on the pivot base, relative the other stationary clamps.
In the welding fixture disclosed in the above-identified publication, the pivot axis about which the pivot base is pivoted is displaced a certain distance away from the pre-welded assembly, in the direction perpendicular to the length of the assembly. More specificaly, the pivot base is adapted to be pivoted about an axis which is located at substantially the same level as the base itself. Since the stationary clamps are supported on the pivot base via a considerably high support block, there is a considerably large distance between the long component members of the pre-welded assembly, and the pivot axis of the pivot base.
In the above arrangement, a pivotal motion of the pivot base will cause not only a change in the level of the stationary clamps on the pivot base, but also a change in the position of these stationary clamps in the longitudinal direction of the component members. For this reason, the welding fixture using this type of pivot base cannot be used for both preliminary and main welding processes, though the fixture can be used to compensate for a slight welding warpage. If the pivot base is pivoted by a suitable angle after the preliminary welding of the component members with the pivot base placed in a preliminary-welding position, the pre-welded assembly can be deflected or bent to compensate for the expected welding warpage due to strains which will occur in the subsequent main welding process. However, the pivotal movement of the pivot base while the pre-welded assembly is clamped by the stationary clamps on the pivot base, will cause a large amount of frictional slip of the stationary clamps relative to the clamped pre-welded assembly, since the position of the stationary clamps is shifted in the longitudinal direction. Accordingly, the clamping units tend to be damaged. Although the frictional slip may be avoided if the pre-welded assembly is unclamped prior to the pivotal movement of the pivot base, and re-clamped after the pivotal movement, it is not desirable or permissible that the clamping positions are changed between the preliminary welding and the main welding, where the component members to be welded are not straight, or where the transverse cross sectional shape of the component members changes in the longitudinal direction. Hence, the welding fixture disclosed in the above-identified document cannot be used for both of the preliminary and main welding operations.